Diffusion bond for refractory metals



2 Sheets-Sheet 1 INVENTORS HAROLD V- HAGADORN 8 BRUCE R. JOHNSTON g4ATTORNEY M. W. HAGADORN ET AL DIFFUSION BOND FOR REFRACTORY METALSOriginal Filed Dec. 11, 1964 May 5, 1979 9 l1 9 W 3 M Hm L 3 3 0 H T L Ma 1 H mm we F w n EM 0" h .H -H u H AA 6 om M RD 4- n ET E TAR K T n) NE5 mmw $L 1 mm M m i m W 3 vl lnars lr l H m .2 l i 5 I 5 mm m A PM rm 3a m m m nnmw mm lllil'l "NH 8 mm m Wm H mwww W. AE A A5 0 T N HMFF Mm FEE A W 08m Rc May 5, 1970 M. w. HAGADORN ET AL 3,510,280

DIFFUSIQN BOND FOR REFRACTORY METALS Original Filed Dec. 11. 1964 2Sheets-Sheet 2 CONTACT REFRACTORY METAL AND A SECOND METAL SURROUND AREAOF CONTHCT BETWEEN SHID ME THLS WITH A LAYER OFA THIRD MEUIL 1 HEAT SAIDMETALS T0 INTEK DIFFUSE SHID THIRD METAL INTO SAID REFRACTORY RAIDSECOND METALS AND FORM A DIFFUSION BOND T COOL 5m METALS J INVENTOR5MAROLD \IV. HAGADORN 8- BRUCE R. JOHNSTON ATTORNEY United States Patent3,510,280 DIFFUSION BOND FOR REFRACTORY METALS Marold Weston Hagadorn,Lima, Ohio, and Donald R.

Kerstetter, Emporium, Pa., assignors to Sylvania Electric Products Inc.,a corporation of Delaware Original application Dec. 11, 1964, Ser. No.417,574, now Patent No. 3,431,631, dated Mar. 11, 1969. Divided and thisapplication Aug. 14, 1968, Ser. No. 752,582

Int. Cl. B32b 15/02 US. Cl. 29196 5 Claims ABSTRACT OF THE DISCLOSURE Ametallic composite consists of a refractory metal, such as tungsten ormolybdenum, diffusion bonded by iron to a second metal such asmolybdenum or nickelplated steel. The refractory metal is initiallysurrounded by a sintered iron sheath which, on heating in contact withthe second metal, melts and forms a diffusion bond between the twometals.

CROSS-REFERENCE TO RELATED APPLICATION This application is a divisionalapplication of Ser. No. 417,574, filed Dec. 11, 1964 now Pat. No.3,431,631, dated Mar. 11, 1969 and assigned to the assignee of thepresent application.

BACKGROUND OF THE INVENTION This invention relates to metal bonds andmore particularly to diffusion bonds between refractory metals anddiffusion bonds between a refractory metal and a nonrefractory metal.

The bonding of refractory metals and of a refractory metal to anonrefractory metal has long been a source of problems and frustrationsin the manufacture of devices such as electron tubes which utilizerelatively large amounts of such materials. Not only does the relativelyhigh melting temperature of refractory metals provide a problem inattaching such metals but the tendency of such metals to recrystallizeand become brittle below the melting temperature thereof provides evenmore difficulties when attempts are made to provide a dependableattachment thereof to other metals and especially other refractorymetals.

As an example, the core materials in the filament of directly heatedelectron discharge devices and the heater in indirectly heated electrondischarge devices is usually of a refractory metal such as tungsten.Also, this core material is frequently attached to a nonrefractory metalsupport member by the application of heat and pressure. Since theapplication of heat and pressure suflicient to melt and bond therefractory metal would destroy the support member, it has been a commonpractice to apply only enough heat and pressure to embed the refractorymetal in the nonrefractory metal. As a result, the attachment is not abonding technique but rather a compromise arrangement which merelysticks one metal to the other.

Further, even though the heat and pressure utilized in this compromisemethod of attachment is insufficient to melt the refractory metal, ithas been found that the refractory metal does recrystallize and becomeembrittled in the area of applied heat and pressure as well as in thearea immediately adjacent thereto. Moreover, these recrystallized andembrittled areas have long been known as the cause of catastrophicfailures in electron tubes.

In attempting to overcome these shortcomings, one approach has been thedeposition of a shim material 3,510,280 Patented May 5, 1970 such asnickel intermediate the refractory metal and the nonrefractory metal.Upon the application of heat and pressure, the two metals are, ineffect, brazed together. However, it has been found that the shimmaterial, such as the above-mentioned nickel, has a tendency to combinewith the refractory metal to form brittle intermetallic compounds. Thus,the results obtainable with such techniques leave much to be desired.

Another example of the use of refractory metals in electron dischargedevices is the fabrication of strapframe type electrodes. In suchelectrodes, a wire helix of refractory metal is usually wrapped about apair of spaced refractory metal side members. Then each turn of thehelix is attached to the side members at the area of contacttherebetween in order to prevent displacement of the turns. A commonmethod for attaching the helix and side members is to surround the areaof contact with a layer of thermosetting ceramic which affixes eachhelix turn to the side member. Although the ceramic serves to attach thehelix and side members reasonably well, it has been found that theceramic renders removal of the helix from the side members mostdifiicult, if not impossible, without damage to the side members. Thus,the use of a ceramic prevents the salvage of usable parts fromelectrodes. Further, the utilization of a ceramic material for afiixingthe metals leaves much to be desired because of the relatively poorelectrical conductivity of the ceramic as compared with the relativelygood electrical conductivity obtainable with a metal.

In each of the above examples, as well as numerous others, asatisfactory solution to the problems of at taching a refractory metalto another refractory metal and to a nonrefractory metal has not beenpreviously provided. Moreover, this problem is especially evident to themanufacturer of electron discharge devices.

OBJECTS AND SUMMARY OF THE INVENTION Therefore, it is an object of thisinvention to enhance the bonding of refractory metals as well as thebonding of a refractory metal and a nonrefractory metal.

Still another object of the invention is to provide an improveddiffusion bond of refractory metals and a refractory metal to anonrefractory metal.

A further object of the invention is to enhance the bonding of arefractory metal core and a nonrefractory metal support in an electrondischarge device.

These and other objects are achieved in one aspect of the invention byattaching a metal sheath to a refractory metal, contacting the sheathand second metal, applying pressure and heat to the sheath and metalssufficient to melt the sheath and interdiffuse the metal thereof intothe refractory metal and the second metal to form a diffusion bond, andremoving the pressure and heat to permit the solidification of thesheath metal.

Alternately, a refractory metal is placed in contact with a secondmetal; a layer of a third metal is affixed to and surrounds the area ofcontact between the refractory and second metal; heat is applied to themetals in an amount sufficient to sinter the third metal and causeinterdifiusion thereof into the refractory and second metals and theformation of a diffusion bond therebetween; and the heat is removed tocause the metals to cool.

For a better undertsanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of adirectly heated electron discharge device having a refractory metal corediffusion bonded to a nonrefractory metal support;

FIG. 2 is an enlarged view of the bonded core and support of FIG. 1;

FIG. 3 is an illustration of the bonding of refractory metal crossmembers to refractory metal side members in a frame suitable for use inthe fabrication of a strapframe type electrode.

FIG. 4 is a flow chart illustrating a process for bonding a refractorymetal and a second metal;

FIG. 5 illustrates a strap-frame type electrode wherein a refractorymetal helix is bonded to a refractory metal side member; and

FIG. 6 is a flow chart illustrating a bonding process applicable to theelectrode of FIG. 5.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawings, FIG. 1illustrates a typical directly heated electron discharge device 7,including a hermetically sealed envelope 9, an anode electrode 11attached to and extending through one end of the envelope 9, and aplurality of electrical conductors 13 sealed into and extending throughthe opposite end of the envelope 7. Disposed within the anode electrode11 is a directly heated filament 15 having a layer 17 of potentiallyemissive materials, such as a mixture of the well-known alkaline earthcarbonates, surrounding and adhered to the major portion of a metal core19. Also, the filament 15 has end portions 21 and 23, respectively,attached to connectors 25 and 27 which are affixed to separate ones ofthe conductors 13. In this manner, the filament 15 is supported andpositioned within the discharge device 7.

Referring to the exploded view of FIG. 2, the filament 15 has a layer 17of potentially emissive materials surrounding a major portion of thecore 19. The core 19 usually and preferably is a refractory metalselected from the group of which tungsten, molybdenum, columbium,platinum, and ruthenium, as well as alloys thereof, are suitableexamples. Also, the connectors 25 and 27 are usually of a material suchas nickel, steel, and alloys thereof although numerous other metals areequally applicable and appropriate so long as they are adapted forattachment of other metals thereto and are not deleterious to theoperation of a discharge device.

Afiixed to and surrounding the end portions 21 and 23, respectively, isa metal sheath 29. The sheath 29 is of a metal characterized by theability thereof to wet other metals, melt at a temperature lower thanthe melting temperature of refractory metals, resist the formation ofbrittle intermetallic compounds upon contact with the refractory metalswhen pressure and heat are applied thereto, and have a slight solubilityin refractory metals. Examples of materials applicable for the metalsheath 29 are metals selected from the group consisting of iron,platinum, ruthenium, rhodium, palladium, osmium, and iridium, as well asplatinurn-copper and silver-copper alloys.

In another application, FIG. 3 illustrates a frame 31 suitable for usein the fabrication of a strap-frame electrode for an electron dischargedevice. The frame 31 includes spaced side members 33 and spaced crossmembers 35. The side members 33 and cross members 35 are preferably of arefractory material such as molybdenum in order to provide sutficientstrength to the frame 31, and a layer 37 of a metal selected from theaboveenumerated group of metals applicable for the metal sheath 29 ofFIG. 2 surrounds and is adhered to the cross members 35. The crossmembers 35 bridge and are diffusion bonded to the side members 33 at ajointure 39 by means of the layer 37.

As to a process for bonding a refractory metal to a second metal,reference is made to the fiow chart illustrated in FIG. 4. Therefractory metal is surrounded by an adherent sheath of metal selectedfrom the previously enumerated group of sheath metal materials havingthe desired and necessary characteristics. Obviously, any one of anumber of well-known techniques for applying a metal sheath to arefractory metal is equally applicable and appropriate. For example, therefractory metal may be dip coated in a slurry of metal particlessuspended in a volatile organic binder and fired to remove the binderand sinter the metal particles to each other and to the metal.

Obviously, the metal particles must be sinterable and the firingatmosphere shoud be reducing in order to prevent the formation ofundesired oxides which would have a deleterious effect on the sinteringprocess. Further, the binder should be organic and any of the ordinarybinder materials are applicable so long as the volatilizationtemperature thereof is lower than the sintering temperature of the metalparticles and no undesired residue remains after volatilization thereof.Thus, a tenacious and stable metal sheath is provided which is neitherharmful to the operation of such critical devices as electron tubes norattacks the metals in contact therewith.

The metal sheath surrounding the refractory metal is placed in contactwith a second metal which may be either another refractory metal or anonrefractory metal suitable for attachment of other metals thereto.Heat and pressure are applied to the sheath and the metals in a quantityand at a rate sufficient to cause the sheath to melt and the metalthereof to interdiffuse into and diffusion bond both the refractorymetal and the second metal. Thereafter, the heat and pressure areremoved and the melted sheath metal cools and solidifies.

As a specific example of the process, the exposed leg portions of atungsten heater for an electron discharge device were dip coated with aslurry which included the following ingredients and proportions:

Carbonyl iron (grade E)10O g. Lacquer-R. and S.40 cc. Pentacetate-ZO cc.

The lacquer is a standard volatile organic binder product available fromRaifi and Swanson Co. located at Chelsea, Mass., and the pentacetate isavailable from the Sharples Corporation and is a solvent which, as faras is known, appears to be a mixture of N-amyl acetate Z-me'thyl butylacetate 3-methyl butyl acetate 4-methyl butyl acetate 3-ethyl propylacetate The heater having slurry-coated leg portions was fired in areducing atmosphere for approximately 10 minutes at a temperature ofabout 800 C. During this firing the organic binder was volatilized andthe iron metal particles sintered to each other and to the tungsten legportions and provided an iron metal sheath thereon.

Following, each leg portion was placed in contact with a nickel-platedsteel electrical connector as is commonly used to provide electricalconnections in electron tubes. Each leg portion and contacting connectorwas then placed intermediate a pair of electrodes in an ordinaryresistance welding apparatus, and heat and pressure were applied theretoin an amount and at a rate such that the iron metal melted and diffusedinto the tungsten and the nickel-plated steel connector to form adiffusion bond therebetween.

Upon removal of the heat and pressure, the melted iron solidified andthere was provided a diffusion bond between the tungsten and the ironand the iron and the nickel-plated steel. Also, the iron was welded tothe nickel-plated steel. Moreover, it was found that the quantity ofheat and pressure required to bond the metals was much less than theheat and pressure ordinarily required to affix a tungsten tonickel-plated steel. Also, microscopic examination revealed that thebond was of a diffusion type, and the tungsten wire showed no evidenceof recrystallization or embrittlement.

'In another example, a molybdenum cross member was bonded to amolybdenum side member of a strap-frame grid electrode in a mannersimilar to the above-described process. The cross member was dip coatedwith the abovelisted slurry, fired to volatilize the binder and sinterthe metal particles, placed in contact with the molybdenum side member,and sufiicient heat and pressure applied to the cross member and sidemember to cause the iron to melt and diffuse into the refractory metalsto form a diffusion bond therebetween. Upon removal of the heat andpressure, the iron solidified and the refractory metal cross member wasbonded to the refractory metal side member.

In an alternate technique, FIG. 5 illustrates a strapframe typeelectrode 41 suitable for use in an electron discharge device. Theelectrode 41 utilizes the strap frame 31 of FIG. 3 which includes theside member 33 and the cross member 35 bridging and bonded thereto.Also, a wire helix 43 is wrapped about the side member 33. The sidemembers 33 and the helix 43 have an area of contact 45 therebetween andeither or both are of a metal selected from the above-mentioned examplesof refractory metals.

Surrounding the area of contact 45 and diffusion bonding the helix 43 tothe side member 33 is a layer 47 of a third metal. This third metal hasall of the previously mentioned characteristics applicable to thematerial for the metal sheath 29 of FIG. 2, such as the ability to wetother metals, resist the formation of brittle intermetallic compoundsupon contact with refractory metals and the application of heat thereto,and have a slight solubility in refractory metals.

Additionally, metals which form a solid solution in another metal andare sinterable at a temperature of not more than about 900 C. areapplicable to the fabrication of electrodes 41 of the strap-frame type.Since electrodes 41 of the strap-frame type are usually fabricated witha helix 43 having a wire with a diameter in the range of about 0.0001 to0.001 inch, it has been found that temperatures greater than 900 C. tendto distort the helix 43, and such distortion deleteriously affects thecritical spacing obtainable with such an electrode 41. Thus, any of thematerials applicable to the metal sheath 29 of FIG. 2 are equallyappropriate as the third metal in a strap-frame electrode 41 so long asthe sintering temperature of the third metal is not greater than about900 C. Further, alloys which include aluminum and nickel in a solidstate solution are also applicable because of the inability of thealuminum and nickel to form brittle intermetallic compounds with arefractory metal while in solution form.

As to a process for fabricating an electrode 41 of the strap-frame type,reference is made to the flow diagram of FIG. 6. The frame 31 of FIG. 3,having refractory metal side members 33, is wrapped with a wire helix 43which is also preferably a refractory metal. Thus, an area of contact 45therebetween is provided, and this area of contact 45 is surrounded by alayer of a third metal. Then the electrode with the layer of the thirdmetal thereon is heated in a reducing atmosphere to a temperature notgreater than about 900 C. whereupon the third metal interdiffuses withthe refractory metals to produce a refractory metal bond. Thereafer, theelectrode is removed from the heat and allowed to cool.

As a specific example, a frame having molybdenum metal side members waswrapped with a wire helix of tungsten wire. A slurry of iron metalparticles suspended in a binder such as previously described was paintedon and surrounded the area of contact between the side members and thewire helix. The electrode was then heated to a temperature of about 800C. for 10 minutes in a dissociated ammonia atmosphere whereupon theorganic materials were volatilized and the iron metal particles sinteredand diffused into the wire helix as well as the side members to form adiffusion bond therebetween. Upon removing the heat, the metals cooledfor subsequent use of the electrode.

Thus, there has been provided a diffusion bond as well as a uniqueprocess for diffusion bonding refractory metals and a refractory metalto a nonrefractory metal which have numerous advantages over prior knownrefractory metal attachments and techniques for attachment. Forinstance, the operational latitude of applied heat and pressure has beengreatly increased and the reliability and repeatability of the bondcorrespondingly enhanced. Further, the metal attachment is a diffusionbond rather than merely an aflixing of one metal to another, and thisdiffusion bond is provided without embrittlement or recrystallization ofthe materials which are bonded.

Additionally, an electrode of the strap-frame type has been providedwherein a refractory metal wire helix and side member are diffusionbonded. Moreover, this diffusion bond is provided by a process whichneither embrittles nor recrystallizes nor causes a distortion orrelaxation of the refractory metals. Thus, the critical spacingadvantages of such an electrode are further enhanced by the improvedattachment between the helix and the side members. Further, the improvedelectrical conductivity between the refractory metals, as well as therelative ease with which the bond can be removed to permit salvage ofusable parts, are believed to be advantages unobtainable by any otherknown technique.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madether in without departing from the scope of the invention as defined bythe appended claims.

We claim:

1. A diffusion bonded metal composite comprising:

a refractory metal;

a second metal adapted for attachment of a metal thereto, and

a third metal of iron particulate sintered and interdiffused into saidrefractory metal and said second metal.

2. The composite of claim 1 wherein said second metal is a refractorymetal.

3. The composite of claim 1 wherein said refractory metal is tungsten.

4. The composite of claim 1 wherein said second metal is nickel-platedsteel.

5. The composite of claim 1 wherein said third metal of iron is in theform of a sheath surrounding said refractory metal.

References Cited UNITED STATES PATENTS 2,844,868 7/1958 Cline et al.29498 3,066,393 12/1962 Malagari 29198 3,188,720 6/1965 Husni 29l98 X L.DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S.Cl. X.R. 29-1966. 198

7; UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N5.3,510,280 Dated May 5, 1970 Inventor) Marold W. Hagadorn and Bruce R.Johnston It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

In sheets 1 and 2 of the drawings, delete Bruce R. Johnston and insertDonald R.

Kerstetter.

smraznam swan I M29191) 6 Meat:

mm H. If, I. JR- A" 0mm Oomisaiom of Patents

